I am not sure you can make that connection between physical appearance and reproductive ability. But in general, although very closely related but very different looking species may be able to have offspring they do not because of ecological barriers(say modern man and something like a Homo erectus: i don't know if they would be able to reproduce, but even if so, I doubt they would.)

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

I agree with MrMistery. While genotype dictates phenotype, you cannot tell from phenotype if the similarity in genotype is sufficient or not for reproduction. As far as actual examples go crosses between individuals that are barely similar (genotypically) to reproduce usually results in sterile offspring. Ex. many plant hybrids, mule (horse + donkey).

I assume, from what you are both saying, that whilst genotype is related fairly strongly to phenotype one cannot assume that phenotype is related to genotype. In other words two similar looking animals may not produce offsprings because they a in fact genetically dissimilar (as in convergent evolution).

But I was posing the question the other way around. The Evo-Devo theory suggests that a small mutation in a single species (phenotype identical) can produce a large difference in phenotype. In other words we should be seeing say a winged animal mating with a non winged animal because they are almost genetically identical but phenotypically very different. But we do not see this sort of thing in the world. Or do we? Are there very different looking animals which can interbreed?

Let me get this straight: you are not talking about different animals (similar or not), you are talking about 'mutant' and ‘normal’ animals breeding? If you are, then your ‘different looking animals’ are just what is called ‘different varieties’ in case of plants and ‘different breeds’ in case of animals. And we do see a lot of those interbreeding. Ex. Two dogs that look nothing alike have no problem breeding. Your example is a bit extreme. I do not see how a simple mutation would cause one animal to acquire characteristic of different species. The more realistic example is having a part of their anatomy duplicated (cows with extra limbs or humans with extra fingers are rare, but do exist), or another one with part of anatomy missing (those usually do not survive long enough). In those cases, if the mutation does not interfere with animal’s ability to reach maturity, then it should not interfere with their ability to mate with regular kind (unless, of course, it is severe enough for regular animal of the same kind to view it as too abnormal or too weak to mate).

Yes, in the "traditional" theory of evolution one would expect a small mutation to produce a small change within the species and so breeding as you say would occur. Now take a few more small mutations and breeding would probably still be possible because we would still be within the same species.

Evo-Devo suggests that just a few mutations in a small area of the geneome can produce very large changes, producing mutants which could not be clasified as belonging to the same species phylogenetically. I do not know how many mutations would be needed to become another species but it appears to be a small number compared to the traditional theory. Therefore if the rest of the genome stays the same, these mutatants looking very, very different and which by observation would be classified as a different species, should be able to breed because their genomes would still be virtually 100% identical.But we do not see this, therefore, however attractive the theory it does not seem to me to fit in with what we observe.

Then we go back to the example I gave originally: horses can and do breed with donkeys to produce mules. They are different species within the same genus and their offspring, mules, are unable to reproduce. Part of being different ‘species’ by definition is that they cannot produce fertile offspring. I would suspect that a lot of closely related species could mate and produce offspring if they ever found themselves in the situation where they do not have their regular mates, but I do not think that is likely to happen often outside of mating barn though.

The examples you give are within the same genus and all look alike. As I understand it (and correct me if I am wrong) the Evo Devo theory suggests that minimal mutations in a very localized region of the genome are responsible for very large changes in appearance. The theory suggest an accelerated way of adapting to new environmental conditions compared to the slow traditional method over hundreds of thousands of years. Because the "switches" which control the form taken by the developing embyo are physically close to each other, this suggests that an almost point mutation can have large effects. So we wouldn't be talking about mules, donkeys and horses but about creatures from within the same species looking very different over a very short period of Time.

With identical genomes (apart from changes in a few switches) should they not still be able to breed? You are suggesting that closely related species could breed given the right conditions, but I am suggesting from the Evo Devo theory that they wouldn't even need to appear to be closely related. Genetically, they would be the same species but to look at they would appear to be completely different species.

Cat is kinda right and kinda wrong: while we define a species as reproductively isolated from all other species in theory, in the practical world we define two different species when they do not reproduce(even though they may have the ability to do so). I remember a case study on 2 species of the genus Pipilo in the US that lived a forest away from one another and did not mate with members of the other species. However, when a logging company cut down the forest 100% vigorous hybrids appeared. So we are forced to make compromises like this, because we simply do not know what species might be able to interbreed. Some have different mating systems, different mating seasons, or look too differently to "be attracted" to one another.

Evo-Devo fits nicely here. Going back to my days of learning molecular development, I can hypothesize about a chicken that has a single point mutation in the gene for the Sonic Hedgehog growth factor. This would cause the Zone of Polarizing Activity directed axis formation in limbs to go wrong, resulting in chickens with very very differently looking wings than the others. In captivity that would probably not be a problem, and without the intervention of the farmer the chicken would probably reproduce. However, in nature natural selection would take over, selecting the new oddly shaped wing(or not) against the classical wing. The possibility of reproduction between the odd chicken and a regular one is not doubted, the question is whether it would be selected or not by the environment as the best one. Also, thinking of some wild chickens and related birds that have their mating rituals intimately tied with wing feather patterns, i would make an educated guess that at least in those species the odd chicken would most likely be left out of the reproductive pool. So, to conclude, i agree with you that two very different looking organisms that formed by a mutation in homeotic genes according to the evo-devo theory most likely CAN interbreed, but they most often won't. The reasons are numerous, with one example being the one above.

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

Thanks - those are interesting points. They fit in with a recent topic "species & race" where the question of racism was touched upon. I suggested that the many species of birds that we find may be due in part to their superb vision which they use to avoid mating with variants, thus accelerating the isolation and hence development of new species.

Now that genome analysis can be performed we should be able to produce hybrids from animals which would appear not to be of the same species. I do not know if this has been attempted, but if it is possible then it would add strength to the Evo Devo theory.

The reluctance to breed with variants within the species paradoxically increases the chances for the variations to breed with other variants and produce further new species. Is this inbuilt reluctance a result of Natural Selection?